The technology to detect and read digital watermarks that are embedded in images is well developed. For example see, U.S. Pat. No. 5,721,788, U.S. Pat. No. 5,745,604, U.S. Pat. No. 5,768,426 and U.S. Pat. No. 5,748,783. Programs for detecting and reading digital watermarks are included in various commercially available image editing programs such as Adobe Photoshop that is marketed by Adobe Corporation.
A digital watermark can more easily be detected and read from a high quality, high resolution image, than from a low quality or low resolution image. In some situations multiple images having similar picture content are available. There are known techniques for combining multiple low resolution images which have similar content in order to make one relatively high resolution image. Such a technique is, for example, described in U.S. Pat. No. 6,208,765. The system shown in U.S. Pat. No. 6,208,765 aligns images using a reference coordinate system. An enhanced image is then synthesized, and regions of image overlap (i.e. regions of similar image content in multiple images) have improved quality. The synthesis process combines information in overlapping regions to form an enhanced image that corrects many of the image impairments.
Inexpensive low resolution cameras designed for connection to personal computers are in widespread use. Such cameras are herein referred to as PC cameras. PC cameras generally capture pixels in what is often termed a “Bayer pattern”. A Bayer pattern is a four pixel square where only one color is captured for each pixel. The colors captured for the two pixels on the first line are red and green. The colors captured for the two pixels on the second line are green and blue. Interpolation is used to calculate three colors for each pixel position. The positions in the Bayer pattern where values of a colors are calculated rather than actually measured are herein termed “holes”.
If a camera which uses pixel interpolation is used to acquire a digital image of a watermarked physical image, the pixel interpolation may make it more difficult to accurately read the watermark from the acquired digital image. However, with cameras such as PC cameras, it is easy to obtain multiple images which have almost identical content. The present invention is directed to using such multiple images to minimize or eliminate the need to interpolate to obtain a high resolution image.